Operating satellite communications on-the-move (“COTM”) requires an antenna system that is compact and reliable in order to facilitate mobile field operations. FIG. 1 illustrates the principal building blocks of a conventional on-the-move terminal. FIG. 1 shows an arrangement of parts typically used to form an on-the-move terminal for satellite communications. Here the antenna is housed inside the radome, and the RF box connected to this radome is the transmitter.
In a traditional reflector antenna, a horn illuminates the reflector surface. To minimize the spill-over effect, the horn is adjusted such that the intensity gradually decreases towards the edges. This condition leads to lower efficiency in terms of gain relative to surface area. There is also the blocking effect of mechanical obstacles that reduces the effective illumination of the surface area. Reflection on the surface itself will also decrease the efficiency due to surface irregularities and other factors, thereby leading to typical reflector efficiencies (defined in percent as the antenna gain relative to the calculated antenna gain for an antenna with the same area with zero losses) in the order of 50-80% depending on geometry, whereas the horn antenna typically will have an efficiency of 80-90%. As the aperture dimension decreases, it becomes more difficult to maintain a high efficiency in the reflector system. This is a result of the fact that illumination problems, irregularities, and blockage effects stay roughly constant while the reflector surface area decreases. Typically the efficiency is hard to keep above 50% as the ratio D/λ˜25, where D is the diameter of a circular antenna and λ is the wavelength.
A typical horn is illustrated in FIG. 2. The illustration shows a horn antenna for Ku-band frequencies. The signal is fed into the antenna on the flange to the right in the figure, and is formed along the horn aperture to the antenna interface on the left.
There is, therefore, an increasing but unmet demand for a small antenna system for COTM with a geostationary or geosynchronous satellite to and from a land mobile, maritime or airborne vehicle that can improve system robustness and simplify handling and integration on the vehicle.